The present invention relates to computer technology, and more specifically, to controlling the read head actuator in a disk drive.
In the above referenced co-pending patent application entitled xe2x80x9cImproved Chatter Reduction in Sliding Mode Control of a Disk Drive Actuator,xe2x80x9d a sliding mode controller (SMC) is employed for actuating a read head over a disk storage medium during seeks and tracking. One advantage provided by SMC is that it is substantially unaffected by parameter variations and external load disturbances in the servo system. Thus, SMC is a more robust control system as compared to more conventional linear control systems. Further, in many instances SMC obviates the need for notch filters to compensate for mechanical resonances typically required in conventional linear controllers. The SMC system of the aforementioned co-pending patent application was derived in continuous-time and the result implemented in discrete-time without loss of generality or performance. Continuous-time SMC operates by switching between two or more feedback systems in order to drive predetermined phase states (or phase state errors) of the system toward a reference point in a phase plane. A phase state trajectory or sliding line "sgr" is defined in the phase plane, and the phase states are driven toward the sliding line "sgr" by the continuous switching action of the SMC.
In continuous-time SMC, the phase states are guaranteed to switch along the sliding line "sgr" when the following xe2x80x9cexistence equationxe2x80x9d is satisfied:             lim              σ        ->        0              ⁢          σ      ⁢                        ⅆ          σ                          ⅆ          t                       less than   0.
As long as the positive and negative feedback gains in the continuous-time SMC are greater than a derived threshold, the inequality in the above existence equation is satisfied. Furthermore, the robustness of continuous-time SMC to parameter variations and external load disturbances increases by increasing the magnitude of the switching gains. However, the extent that the switching gains can be increased is limited by the constraints of the control effort (e.g., the drive current) and the desired power dissipation.
Another problem with continuous-time SMC is that the switching action can generate undesirable acoustic and electromechanical xe2x80x9cchatterxe2x80x9d in the system due to the phase states frequently crossing the sliding line "sgr". The aforementioned co-pending patent application discloses a technique for chatter reduction by defining a converging boundary layer around the sliding line "sgr". This modification reduces the frequency that the SMC switches between the positive and negative feedback gains by disabling the switching action while the phase states are within the boundary layer. Although this technique reduces chatter, it may be sub-optimal depending on the system dynamics and desired response since it limits the maximum slope of the sliding line "sgr". In addition, this technique still may require a large control effort to maintain the desired robustness to parameter variations.
There is, therefore, the need for an improved SMC for controlling the motion of a read head in a disk storage system that overcomes the drawbacks of SMC implemented using the above existence equation. In particular, there is a need for an improved SMC that requires less control effort and that produces less switching noise while still providing sufficient immunity to parameter variations and external load disturbances.
A discrete-time sliding mode controller (SMC) is disclosed for controlling the motion of a read head actuated over a disk storage medium during tracking operations. The discrete-time SMC comprises a linear signal generator for generating a linear control signal, and a sliding mode generator for generating a sliding mode signal. These signals are combined and applied to a voice coil motor (VCM) for positioning the read head over a particular data track recorded on the disk. The sliding mode control signal is generated according to
(cxcex93)xe2x88x921xcex94sgn("sgr"k)
where (cxcex93)xe2x88x921xcex94 is a predetermined gain and "sgr"k is a sliding line variable that represents the position of the system phase states relative to a phase state trajectory or sliding line in a phase plane. The linear signal generator and the gain (cxcex93)xe2x88x921xcex94 are designed such that once the sliding mode variable "sgr"k crosses the sliding line the first time, it will cross the sliding line in every successive sample period resulting in a zigzag motion about the sliding line wherein "sgr"k changes sign at every sample period. The linear controller and the gain (c"sgr")xe2x88x921xcex94 are also designed so as to constrain "sgr"k to
"sgr"3+1=xcex"sgr"kxe2x88x92xcex94sgn("sgr"k)
where xcex and xcex94 are predetermined constants and xcex less than 1. The above equation ensures a quasi-sliding mode condition wherein the magnitude of the sliding mode variable "sgr"(x) is substantially constrained to a boundary layer
|"sgr"k|xe2x89xa6xcex94/(1+xcex).
In this manner, the switching action and resulting chatter of the sliding mode controller are minimized by adjusting the width of the boundary layer through appropriate selection of xcex94 and xcex. In fact, the boundary layer can be made arbitrarily small so as to better approximate and thereby attain the benefits of continuous time SMC. In addition, the required control effort is minimized while still providing adequate immunity to parameter variations and external load disturbances.